A display device such as a television cathode-ray tube comprises a glass envelope consisting of a faceplate and a funnel-shaped rear part. When the tube is a tube for reproducing colour images, a luminescent screen is placed on the inner surface of the faceplate, the said screen comprising three phosphor arrays corresponding to the three primary colours red, green and blue. An electron gun is placed at the back of the tube, inside a cylindrical neck, in order to generate one or more beams for scanning the screen under the effect of magnetic fields created by a deflection device placed around the tube at the output of the electron gun.
An electron gun for a cathode-ray tube comprises a succession of electrodes for accelerating and shaping the electron beam or beams emitted from one or more emissive cathodes; these electrodes are placed successively along a longitudinal axis.
For tubes designed to reproduce colour images, the gun generally comprises three cathodes placed in line in one and the same direction perpendicular to the longitudinal axis.
Each cathode is placed inside an eyelet to which it is welded. The eyelet, and therefore the cathode, is held in place by means of a metal piece to which it is welded, the said piece comprising two arms whose ends are inserted into two preheated glass beads extending along the longitudinal axis of the gun, the said beads also serving to hold the various electrodes of the gun one with respect to the other.
The electron gun is inserted into the back of the tube into a cylindrical neck whose diameter is minimum so as to optimize the sensitivity of the electron beams to the fields created by the magnetic deflection device.
The cathodes placed in line in one and the same direction have, in this direction, an overall size greater than that of the glass beads in this same direction; in the prior art as, for example, described in U.S. Pat. No. 4,151,441, the arms supporting the side eyelets are identical but different from the arms supporting the central eyelet. It follows that when operating the tube the mechanical and thermal behaviours of the side and central cathodes will not be identical; for example, since the thermal inertia of the central and side supports are different because of the different volumes of metal, the temperature of the cathodes will rise at different rates and therefore reach their nominal rating at different times, thereby causing discolorations of the image during the transient period after the tube is switched on.